Electret pressure transducer

ABSTRACT

An electret pressure transducer consisting of a very thin insulating coating applied to the surface of a body to be investigated when under stress, a thin layer of conductive paint or metallic deposit configured to extend over the area to be investigated, a layer of a solid elastic material, the electretfilm containing a permanent electric charge, and a metallic deposit on the electret film. The layer of conductive paint and the metallic deposit constitute the two plates of a parallelplate capacitor, and the elastic material in conjunction with the electric charge provided by the electret determines the sensitivity, linearity, and frequency response of the transducer.

United States Patent 1 1 Crites 1 May 29, 1973 1 ELECTRET PRESSURE TRANSDUCER Primary ExaminerStanley M. Urynowicz, Jr. [75 1 Inventor Rger Cmes Flor'ssam Att0rney-Frederick M. Woodruff and Edward A. [73] Assignee: McDonnell Douglas Corporations, Boeschenstein St. Louis, Mo. 57 ABSTRACT [22] Filed: Nov. 4, 1971 l l An electret pressure transducer consisting of a very [21] Appl' l95750 thin insulating coating applied to the surface ofa body to be investigated when under stress, a thin layer of [52] US. Cl. ..307/88 ET, 179/111 E, 317/262 F conductive paint or metallic deposit configured to ex- [51] Int. Cl ..H'0lg 7/02, H04r 19/00 tend over the area to be investigated, a layer of a solid [58] Field of Search ..307/88 ET; elastic material, the electret-film containing a per- 179/111 111 17/262 F manent electric charge, and a metallic deposit on the electret film. The layer of conductive paint and the [56] e e ence Cited metallic deposit constitute the two plates of a parallelplate capacitor, and the elastic material in conjunction UNITED STATES PATENTS with the electric charge provided by the electret deterl,746,540 2/1930 Kyle ..179/111 R mines the sensitivity, linearity, and frequency response 3,118,979 1/1964 Sessler et al. ...179/111 R of the transducer. 3,305,638 2/1967 Teachout ...l79/l1l R 3,300,585 1/1967 Reedyk et a1... ...179/|11 R 7 Claims, 4 Drawing Figures 3,612,778 10 1971 Murphy ..307/88 ET BACKGROUND OF THE INVENTION An electret device is generally understood to be a dielectric body in which a permanent electric charge has been established. Early usage of electrets has been made in microphones as a transducer for converting the pressure generated by sound waves on a diaphragm into electrical signals for reproduction in audio systems. The common electret microphone was constructed in a manner that operated in response to changes of pressure in a trapped volume of air.

The typical electret microphone of the prior art using a trapped volume of air is far too bulky and not at all adaptable to large surface areas that require pressure response investigation, and are incapable of use on surfaces that have compound curvature characteristics. The early electret involved use of dielectric materials that exhibited the electret phenomena, but almost all the materials producing electrets are unstable. That is to say the electric field producedthereby decayed rapidly to an insignificant intensity within a few weeks. Since the early work with electrets was almost entirely with microphones, it limited the thinking to the con cept that electrets should be flat and used with a volume of trapped air so that high sensitivity could be obtained through large changes in the air gap for very small changes in the external pressure.

SUMMARY OF THE INVENTION This invention relates to improvements in electret pressure responsive transducers, and more specifically to transducers that have very thin, free-surface characteristics adaptable to a broad range of uses for measuring fluctuating pressures or detecting sudden changes in fluctuating pressure intensity, or where any small, large, flat or highly curved surface, or where the transducers must not interfere with the flow of a fluid relative to a body.

The transducer herein consists of a thin film body in which the prior art air gap between the electret and a backplate is replaced by a solid elastic material which provides an improved range of dynamic characteristics by the thickness and stiffness of the elastic substrate. In a preferred form of the improvement, the transducer includes a very thin film of electrical insulating material, a very thin layer of conductive material such as paint or vacuum deposited metal, a layer of elastic solid material, the electret material such as FEP Teflon or K] Polycarbonate, and a metallization film on the electret. The transducer is intended to be incorporated in an impedence-matching electronic circuit.

The objects of this invention are to provide an improved transducer employing a thin film electret having a high order of sensitivity to fluctuating pressure, to provide a transducer with an elastic solid state electret film capable of use on large area surfaces having compound curvature characteristics, to provide an electret transducer that can measure sound pressure levels of a very low decible value, and to provide an electret transducer having a linear response to sound pressure and improved frequency response through control of the electret substrate thickness and elastic modulus.

Other objects and advantages of this invention will appear from the following description of a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of this invention and its circuit application is shown in the accompanying drawings wherein:

FIG. 1 is a perspective view of a body to be investigated for surface pressure fluctuations showing a typical electret transducer installation;

FIG. 2 is a sectional elevational view, greatly exaggerated, of a typical improved transducer taken at line 2-2 in FIG. 1 and embodying a solid state electret in association with an electrically conductive body;

FIG. 3 is a sectional elevational view similar to FIG. 2 but showing a modification involving an electrically insulative body; and

FIG. 4 is a diagram of an irnpedence matching electronic circuit for the electret transducer of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electret is a dielectric material which produces an electrical field about itself in the absence of an external voltage source. In this manner, an electret is analogous to a magnet, which produces a magnetic field about itself in the absence of an external current source. Electrets are made by imposing a permanent electrostatic charge onto the surfaces of a dielectric material.

Heretofore electrets have been widely used in microphones and have been designed invariably utilizing a thin film of fluorocarbon, such as FEP Teflon or other fihns such as K-l Polycarbonate metalized on one side. The metal deposit on the electret serves as one conductor of a parallel plate capacitor. An electret of this character is generally stretched over a metallic backplate, thus trapping a thin layer of air, and the metal backplate serves as the other conductor in the capacitor. If the thickness of the electret material is neglected (for clarity) the capacitance of this arrangement is given by the relationship:

In the above relationship e is the electric permittivity of free space, A is the surface area of a capacitor, and d is the distance between the two parallel conductors which is essentially the thickness of the air gap in electret transducers heretofore designed.

As the pressure over the electret fluctuates, the air gap distance d fluctuates because the electret film essentially has no stiffness, whereby the distance d must change just enough for the pressure in the air gap to equal the external pressure acting on the electret at each instance in time. The capactiance will, therefore, fluctuate as a function of the external pressure. The electret emits a constant electric field that is produced by a constant charge O, which, together with the value of capacitance C, determines the voltage developed across the output terminals according to the definition:

The voltage, thereof, will fluctuate as the capacitance C fluctuates, which also will fluctuate as the distance between the two parallel conductors d fluctuates, which, in turn, fluctuates as the external pressure fluctuates. It is understood from the foregoing relationships that voltage developed across the output terminals of the electret capacitor is a function of the pressure acting on the electret surface.

The present electret pressure fiansducer is an improvement upon'the previous devices as it does away with the air gap and incorporates a solid elastomer or elastic material which imparts to the resulting transducer extreme flexibility and broadens the field of application to many more uses. The dynamic characteristics of the solid elastic material electret transducer are determined by the thickness and elastic modulus of the elastic substrate. A typical installation of the improved transducer is shown in FIG. 1 where a group 10, 11, 12 and 13 is applied on an electrically conductive metallic body 14 upon which it is desired to measure fluctuating pressure. Each transducer 10, 11, 12 and 13 is secured in place on the body 14 with leads 15, 16, 17 and 18 supported on the body and extending to a contact pickup pin suitably mounted in an insulated terminal pot 19. A common ground lead 20 is in contact with the body 14, and jumper leads 21, 22, 23 and 24 connect the respective transducers 10, 11, 12 and 13 to this ground. The arrangement shown in FIG. 1 is for obtaining individual reading of each transducer. However, if the pick-up pins in the terminal pot 19 are connected together a composite reading can be obtained which would be the average reading for the total area covered by the group of transducers. Thickness of transducers shown in the figure are greatly exaggerated.

Turning now to FIG. 2 the transducer 10 which is typical of the others comprises a thin film or coating 2 of electrical insulating paint which is applied to the surface of the body 14. Adjacent the electrical insulating film is a very thin film 3 of conductive paint which may be vacuum-deposited material to form one electrode in the electrical capacitor. A solid elastic material substrate 4 is in contact with the conductive film 3 and replaces the prior art air gap forming means between the electret metalization film 3 and the film 5 which is the electret material which may comprise a fluorocarbon film or other suitable films. The other conductor of the electret capacitor is a metalized film 6 on the electret material film 5. In the transducer thus described the electret metalizing film 6 is grounded by jumper lead 21 to the ground lead on the conductive body 14 whereby the conductive film 3 is shielded from stray electric fields and the signal is then taken between film 3 and ground 6 as indicated at the output terminals 7. The electret transducer 10 of FIG. 2 undergoes capacitance changes as a linear function of the compression response of the elastic substrate layer 4. The charge on the capacitor remains constant, as determined by the electret material 5, so that the voltage across the capacitor must fluctuate as the pressure fluctuates, generating a fluctuating voltage without any power supply.

In FIG. 3 the electret pressure transducer 10A is shown as applied to a body or a surface 8 which is electrically insulating. In this arrangement one additional layer or film of conductive material 9 is necessary in order to provide a bottom film layer which may be a very thin metalization. In other respectsthe view of FIG. 3 shows the stacked arrangement of layers 2, 3, 4, 5 and 6 described in connection with FIG. 2.

Electret transducers of the type described in FIGS. 2 and 3 are inherently high-impedance devices. It is necessary, therefore, to utilize an electronic follower which has impedance matching characteristics. A suitable example is a simple FET source follower with more than 10 ohms imput impedance and such a circuit is shown in FIG. 4. The electronic follower circuit is connected at terminal 7 to the transducer 10 or 10A and includes a FET transistor 22, diode 23 and resistor 24 in the negative power return lead 25. The output terminals 26 are connected to any suitable amplifier (not necessary to show). Calibration of the elastic substrate electret transducer heretofore described may be carried out in the reflected wave tube, commercially employed for the calibration of conventional fluctuating pressure transducers.

It has been found that the sensitivity of the electret transducers of this invention is typically two to three orders of magnitude greater than conventional fluctuating pressure transducers. The herein improved electret transducers are capable of measuring further pressure levels 40 to 60 decibels (dB) lower than conventional transducers at the same signal-to-noise ratio, and 10 to 20 dB higher pressure levels than prior an electret microphones. This makes the improved electret suitable for application in low speed wind tunnels where sound pressure levels are commonly less than dB, as well as supersonic wind tunnels with sound pressure levels as high as dB.

Electret pressure transducers using a solid electric substrate or solid elastomer as heretofore described have been developed where the transducer has a thickness of approximately 1 mil. and results in dynamic characteristics which are independent of sensor surface area configuration. Because of this thickness the present transducers are well suited to the measurement of fluctuating pressures over any aerodynamic surface of interest either small or large, flat or highly curved. The sensor area may be easily configured into narrow thin strips for the establishment of boundary layer transition and areas for investigation. Furthermore, the transducer may be produced as an integral tape in which all of the layers are combined with the layer 2 or 9 having adhesive characteristics for direct application to the bodies 14 or 8 as heretofore noted. Also, the electret film may be consolidated with the solid elastic substrate. When so produced the transducer must have the metallic shielding 3 and 6 on each surface of the electret material. The solid elastomer or solid elastic substrate 4 may be separate or combined with the electret.

The transducer heretofore described may also be one large area to provide direct measurement of the integral or average of fluctuation of pressure over that area for buffet or aerodynamic force study. Electret transducers herein described can have resonant frequencies in excess of 100 KH regardless of size or shape, and they have demonstrated excellent linearity, acceptable frequency response, and a dynamic sensitivity which exceeds that of most conventional fluctuating pressure transducers by more than two orders of magnitude. A particularly important characteristic of the present electret pressure transducer is that the material costs involved are negligible, so that the use thereof in place of conventional instruments provides a substantial cost saving.

The present electret pressure transducer provides cheap, high response, fluctuating pressure instrumentation for aerodynamic tests, and may be applied to small or large aerodynamic surfaces, subject to fluctuating pressure forces which require investigation.

The improved transduction mechanism of this electret transducer can be used in an electrostatic speaker,

and because it requires no high voltage bias supply it is more compatable with modern low voltage transistor audio circuits than other electrostatic speakers. Also, because it uses no air gaps it is better suited to high sound level output, particularly in the ultrasonic frequency domain.

The electret pressure transducer herein above described clearly expands the use thereof so that it may be incorporated, for example, in ultra-sensitive intruder alarms, micro-switches, ultra-thin keyboards, chamber pressure monitors for small arms, and particle and hail detectors.

The field of use is also expanded by the present electret pressure transducers to include medical uses involving vital function alarms for intensive care patents and baby incubators; implantable pressure sensors; orthopedic responses and bio-switches which are made to respond to various controllable muscles of the human body, thereby permitting victims of paralysis to control servo mechanisms.

What is claimed is:

1. An electret pressure transducer comprising a body consisting of an electret film adjacent an imperforate solid elastic substrate, and said film and substrate sandwiched between imperforate conductive layers, the film, substrate and layers constituting a parallel plate capacitor in which the thickness of said substrate fluctuates as the applied pressure fluctuates, while the effective electrical charge, due to the electret, remains substantially constant.

2. The transducer set forth in claim 1 wherein said electret film, substrate and conductive layers are flexible and capable of conforming to the shape of a body under investigation for applied pressure.

3. The transducer set forth in claim 1 wherein said conductive layers are applied conductive films, having a combined thickness of about one mil.

4. The transducer set forth in claim 1 wherein said film, substrate and layers have a substantially coextensive area and are in intimate contact.

5. As an article of manufacture in the form of an electret pressure transducer body for placement on an imperforate surface to be investigated for applied surface pressure, said body being subject to the applied pressure and consisting at least of two electrically conductive imperforate and flexible layers and an intervening layer of a compliant imperforate material, said compliant layer containing an electric charge and responding to applied pressure to change the capacitance of said article as a function of the response of said compliant layer to pressure applied on the surface to be investigated, and said electrically conductive layers and said compliant layer being in surface to surface contact.

6. As an article of manufacture set forth in claim 5, the electret pressure transducer thereof in which the plurality of layers in stacked engagement include an electrical insulating coating.

7. The article of manufacture set forth in claim 5 wherein said electret pressure transducer body is flexible and conformable to the configuration of the surface area to be investigated. 

2. The transducer set forth in claim 1 wherein said electret film, substrate and conductive layers are flexible and capable of conforming to the shape of a body under investigation for applied pressure.
 3. The transducer set forth in claim 1 wherein said conductive layers are applied conductive films, having a combined thickness of about one mil.
 4. The transducer set forth in claim 1 wherein said film, substrate and layers have a substantially coextensive area and are in intimate contact.
 5. As an article of manufacture in the form of an electret pressure transducer body for placement on an imperforate surface to be investigated for applied surface pressure, said body being subject to the applied pressure and consisting at least of two electrically conductive imperforate and flexible layers and an intervening layer of a compliant imperforate material, said compliant layer containing an electric charge and responding to applied pressure to change the capacitance of said article as a function of the response of said compliant layer to pressure applied on the surface to be investigated, and said electrically conductive layers and said compliant layer being in surface to surface contact.
 6. As an article of manufacture set forth in claim 5, the electret pressure transducer thereof in which the plurality of layers in stacked engagement include an electrical insulating coating.
 7. The article of manufacture set forth in claim 5 wherein said electret pressure transducer body is flexible and conformable to the configuration of the surface area to be investigated. 